A laser device that emits infrared rays is in wide use such as in environmental measurement. Among such laser devices is a quantum cascade laser (QCL) composed of semiconductors, which is small and convenient, and enables high-accuracy measurements.
A quantum cascade laser has an active layer with, for example, alternately stacked GaInAs and AlInAs, and containing a quantum well layer. The structure of such a quantum cascade laser is such that the active layer is sandwiched by, for example, an InP cladding layer on the both side surfaces. In such a structure, the cascade-connected quantum well layer can emit an infrared laser light of 3 to 20 μm wavelengths as the carrier undergoes an intersubband transition.
For example, the CO2 gas in the breath has an intrinsic absorption spectrum under infrared irradiation. The gas concentration can thus be found by measuring the amount of infrared absorption.
Measuring the isotope ratio of 13CO2 and 12CO2 also enables diagnosis of human body for the presence or absence of any abnormality. In this case, the wavelength of the laser beam emitted by the quantum cascade laser needs to be controlled within a 3.5 to 4.5 μm range.